1. Related Field
The invention relates to an extended nip roll, to an extended nip press that comprises the inventive extended nip roll and to a paper making machine comprising such an extended nip press. The invention also relates to a method of operating the inventive extended nip wherein a wet paper web is passed through the extended nip.
2. Description of Related Art
The invention relates to the field of extended nip presses used in paper making machines. In a paper making machine, an extended nip press is normally used for pressing water out of a newly formed wet fibrous web but an extended nip may also be used for other purposes in a paper making machine, e.g. calendering. Although extended nip presses were first introduced for heavy grades such as paperboard, they later come to be used also for lighter grades such as printing paper. In recent years, such presses have also been used in machines for making tissue paper. In such machines, the extended nip press is often formed by an extended nip roll and a Yankee drying cylinder that acts as a counter roll for the extended nip roll. An extended nip press is typically formed by an extended nip roll that comprises a rigid shoe with a concave surface. The rigid shoe of such an extended nip roll can be made of a material such as, for example, steel or aluminum. U.S. Pat. No. 7,527,708 discloses how an extended nip can be formed by means of a device that does not use a steel shoe but instead a support body which is elastically deformable. The support body disclosed in that document has internal pressure chambers that can be connected to a pressure medium source.
Generally, it is desirable that the pressure in the nip of an extended nip press rises from the beginning of the nip to reach a peak at the end of the nip. Such a pressure profile is advantageous since it reduces re-wetting of the paper web when the web exits from the press nip. In U.S. Pat. No. 7,527,708, it is suggested that different pressure chambers are set under different pressures such that a pressure curve describing a stepped course is obtained.
It is an object of the present invention to provide an extended nip press that is designed in such a way that a suitable pressure profile is obtained.
Another object of the invention is to provide an extended nip press in which the lubrication functions in a reliable way.